Passenger cars and trucks on U.S. highways wear out tens of millions of tires each year. Disposal of these used tires has become a major environmental problem. A high proportion (up to 30-40%) of the weight of a used tire consists of carbon black reinforcing of the rubber in both the tread and sidewalls. This carbon black is prepared by conventional carbon black production processes and comprises individual particles one micron or less in diameter. Fifty to sixty percent (50-60%) of the weight of a discarded tire is butadiene-styrene copolymer rubber. Tires also contain large amounts of oil and significant quantities of steel, wire and/or fiberglass or polyester cord. All of these components are expensive and require large amounts of energy in their manufacture. A process that would allow economic recovery of these materials from the huge stocks of used tires piling up around the country would be very desirable. Unfortunately, the very characteristics that make tires long-lasting and safe of the road, i.e., durability, resistance to puncture and slicing, and resistance to decomposition at moderate temperature, combine to make tires exceptionally dificult to recycle.
The prior art teaches that rubber can be pyrolyzed in the absence of air at temperatures of between 842.degree. and 1112.degree. Fahrenheit in laboratory equipment to produce oil, gas and solid residue that is carbonaceous in nature. Large electrically heated sink reactors and Dewar flasks have been used for obtaining test data.
The prior art also teaches some pilot plants that were built to carry tire processing schemes into the commercial world. Circulating heated ceramic balls have been used a direct source of reaction heat. The balls are heated externally, mixed with rubber feed chips, discharged, screened, reheated and recycled. These reactions take place substantially at atmospheric pressure. Other pilot plants have been designed which make the carbonaceous solid phase of tire pyrolysis into fuel briquets. These fuel briquets are much less valuable than the carbon black produced by the present invention. Still other batch pilot plants have been built in which the tires are indirectly heated through the tray walls of multi-tray reactors to temperatures of between 1400.degree. and 1600.degree. Fahrenheit. At these temperatures, heavy oils and tar products can be recycled for further cracking to improve carbon black yields. Other batch and continuous type process plants have been built that depend on indirect heating through walls of a jacketed screw reactor from a high temperature molten salt heat sink. Other continuous type process plants have been built that depend on indirect heating through hollow shaft and hollow flight screw conveyors from a high temperature molten salt bath, which also use the carbon black for commercial purposes as carbon black.
In each of the prior plants set out above, the heat must be transferred indirectly from a heat source to a solid tire particle through a wall or directly through heated ceramic balls. Indirect heating as the sole means of heat transfer causes coating and other nonuniform heating problems.
It is an object of the present invention to overcome these problems, and heat the tires directly without direct physical intrusion into the process reactor.
An alternate objective of the present invention is to teach a method of and teach apparatus for pyrolyzing used tires economically into commercial quantities of char which can be used as a solid fuel in lieu of carbon black.
Also, in each of the prior art types of pyrolysis plants set out above, the tires were required to be physically broken apart into smaller pieces or fragments except those that show direct melting of held whole tires. Commercially available tire disintegrators include slicing machines, hammer mills, debeaders and manglers that have been adapted to tire reduction from other industries. The recent introduction of steel reinforcing in both passenger and truck tires has greatly increased the difficulty and expense of sufficiently disintegrating a tire to convert it into a useable pyrolysis feed stock.
It is another object of the present invention to overcome the physical difficulties of the prior art in preparing used tires as a feed stock by processing whole tires.
It is also believed that the prior art has never taught a satisfactory method of completely devolatizing the heavy oil and tars that coat the residue carbon black particles due to the limitations of indirect heating or contact direct heating.
It is a further object of the present invention to teach a method of direct heating that fully devolatizes the oil at the carbon black particle thus producing a carbon black with properties equal to that of the original carbon black.
It is yet another object of the present invention to teach a method and an apparatus for pyrolyzing used tires economically into commercial quantities of oil and fuel gas.
It is still a further object of this invention to teach a method of recovering steel scrap from used tires.
It is yet another object of the present invention to teach a method of pyrolyzing used tires that is energy efficient and generates fuel gas necessary to operate a large part of the process within environmental regulations from the process itself.
It is yet another object of the present invention to teach a method of and teach apparatus for pyrolyzing used tires economically into commercial quality and quantities of carbon black.
It is yet another alternate objective of the present invention to teach a method of and apparatus for cooling the hot char, steel and fiberglass while simultaneously providing a means of separating the char from the steel and fiberglass.
It is yet another alternate objective of the present invention to teach a method of and apparatus for reheating the cooled char with an indirect heat source such as hot oil. The heated char is subsequently pyrolyzed with microwave energy to produce carbon black.